A hot swap circuit applies power from an input source to a load in a controlled and protected fashion. One function of such a controller is to limit inrush currents from the power source to the load, especially load capacitance, when power is first applied or if the power source voltage suddenly increases. Another function is to limit current if the load attempts to draw too much current, for example if there is a short circuit in the load.
FIG. 1 shows a hot swap circuit that uses a single MOSFET 100 (Q1) in series with a current sense resistor 102 along with control circuitry for limiting current. Numerous such circuits are commercially available. When limiting current, a current limit amplifier 106 adjusts the MOSFET gate to source voltage in order to limit the voltage across the current sense resistor 102 and thus the current through the MOSFET 100. The current limit amplifier 106 compares a voltage representing the current in the current sense resistor 102 with a voltage VLIMIT produced by a voltage source 104 to control the gate of the MOSFET 100 so as to reduce the output current when the sensed current exceeds a maximum value established by the voltage VLIMIT. A current source 108 is provided for pulling up the gate voltage. A transistor 110 is provided for turning the hot swap circuit on or off.
During this time, the voltage and current through the MOSFET can both be large, resulting in high power dissipation in the MOSFET. If this power dissipation persists, the MOSFET can reach temperatures that cause damage. MOSFET manufacturers present the safe limits on MOSFET voltage, current, and time as a curve referred to as the Safe Operating Area (SOA). Commonly, a timer circuit sets a maximum time for the MOSFET to operate in a current limit mode. When this time expires, the MOSFET is turned off to protect it from overheating. The load will lose power and the hot swap controller will indicate that a fault has occurred. The timer circuit may include a timer capacitor CTIMER coupled to a 2 μA current source 112, which via a switch S1 is coupled to a 100 μA current source 114. The switch S1 is controlled by a control signal produced at the status node of the current limit amplifier 106 that indicates whenever the current limit amplifier 106 limits the current.
Often high power hot swap applications need to charge large bypass capacitors 126 (CL) across the load. To reduce stress on the MOSFET 100, the load may be kept off until the bypass capacitors 126 are charged. A small charging current for the capacitance keeps the power in the MOSFET 100 low enough to prevent a dangerous rise in temperature.
However, in the method described above, the timer runs at an equal rate any time the circuit is in a current limit mode. The timer time-out, at a minimum, must be set to allow the circuit to completely charge the bypass capacitor 126 from ground. An even longer time-out setting may be required if another allowable operating condition, such as a fast increase in the input supply voltage or the presence of a load current during start-up, causes an even longer duration current-limit event. A MOSFET 100 must be selected that can withstand the worst-case SOA condition that occurs during any possible normal operating condition or fault condition. Fault conditions may include events such as start-up into a short circuit that will result in the entire supply voltage being imposed across the MOSFET 100 for the time-out duration. This is a fault condition that requires a greater SOA of the MOSFET 100 than any normal operating condition.
With the previously described method, the worst-case SOA condition occurs during a fault condition, and the customer must select a MOSFET 100 that survives this condition. The worst-case condition is not always readily apparent, and determining the worst-case condition is sometimes the most challenging aspect of designing a hot swap circuit.
Therefore, there is a need for circuit and methodology for MOSFET protection that would overcome the disadvantages discussed above.